This application relates generally to fiber-optic communications. This application relates more specifically to methods and systems for tunable demultiplexing of optical signals.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.
In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronicsxe2x80x94typically an electronic SONET/SDH system. However, SONET/SDH systems are designed to process only a single optical channel. Multi-wavelength systems would require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology.
The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called xe2x80x9cwavelength routing networksxe2x80x9d or xe2x80x9coptical transport networksxe2x80x9d (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. The ultimate connection of individual channels to their destination points requires that the multiplexed optical signal be demultiplexed. Such demultiplexing may also be needed for other specific applications. Currently, passive optical elements are used to impose a fixed relationship between the component wavelengths of the DWDM signal and the physical output ports. To change the relationship of wavelengths to output ports with such a passive system requires replacing the passive optical arrangement. There are various circumstances under which it is desirable to change this relationship, and accordingly there is a general need in the art for a DWDM demultiplexer that may tune this relationship dynamically for individual applications.
Embodiments of the invention thus provide a tunable demultiplexer. The tunable demultiplexer accepts an input optical signal that has a plurality of spectral bands and provides a plurality of output signals. Each of the output signals corresponds to a selected one of the spectral bands. The tunable demultiplexer has at least one wavelength routing element (xe2x80x9cWRExe2x80x9d) and at least one optical arrangement disposed to exchange light with the WRE. The WRE is of the type adapted for selectively routing wavelength components of a first optical signal onto a plurality of second optical signals according to a configurable state of the WRE. The correspondence of a subset of the spectral bands to the plurality of output signals may be determined by a state of the optical arrangement(s) and/or a state of the WRE(s). The WRE may generally be of any type, including a four-pass or two-pass WRE.
The optical arrangement(s) may have a variety of forms in different embodiments and may include such elements as passive filter elements, tunable filter elements, fixed wavelength band filters, tunable wavelength band filters, optical power splitters, optical interleavers, and optical combiners, among others. In certain embodiments, the optical arrangement(s) may also include one or more WREs. In one such embodiment, the WREs are arranged as a tree.
In one embodiment, the optical arrangement includes a plurality of serial arrangements of tunable wavelength band filters, each adapted to provide a first output that transmits a selected filtered portion of a received optical signal and a second output that transmits a remaining portion of the received optical signal. At least one of the tunable wavelength band filters in each of the serial arrangements is configured to received an equivalent to one of the second optical signals provided by the WRE, such as may be provided through the use of an optical splitter.
In another embodiment, the optical arrangement includes an optical power splitter and a plurality of tunable filters disposed to receive light from the optical power splitter. The optical power splitter may be disposed to receive one of the second optical signals from the WRE. The tunable filters may comprise one or more tunable narrowband filters or may comprise one or more tunable wideband filters. In a specific embodiment, the plurality of tunable filters comprises at least one pair of tunable wideband filters tuned with a narrow frequency overlap.
In further embodiments, the optical arrangement includes at least one optical space switch. In one such embodiment, a plurality of WREs are used, each WRE being disposed to receive an equivalent to the input optical signal and configured to route distinct subsets of desired spectral bands to respective filter elements. The filter elements are then configured to transmit individual spectral bands to the optical space switch(es). In one embodiment that uses a plurality of optical space switches, each optical space switch is associated with one of the WREs and the optical arrangement includes a plurality of optical combiners. Each optical combiner receives optical signals from each of the optical space switches and transmits an optical signal corresponding to one of the output signals.
In another embodiment that uses an optical space switch, the optical arrangement has at least one optical interleaver disposed to receive one of the second optical signals from the WRE. A plurality of filter elements are disposed to receive optical signals from the optical interleaver and to transmit optical signals to the optical space switch. A plurality of optical interleavers may be arranged as a tree in one embodiment.
Instead of using an optical space switch, in some embodiments the output signals are electrical signals and an electrical space switch is used. In one such embodiment, a plurality of WREs are used with each WRE being disposed to receive an equivalent to the input optical signal and to route distinct subsets of the desired spectral bands to respective filter elements. The filter elements are configured to transmit individual spectral bands to receivers for conversion to electrical signals. The electrical signals are then provided to the electrical space switch.
In still another embodiment, the optical arrangement includes a plurality of filter arrangements. Each filter arrangement has a plurality of tunable filters arranged serially and disposed to receive an equivalent to one of the second optical signals from the WRE.
In still a further embodiment, the optical arrangement includes a passive wideband filter.